The present invention relates to an audio/video synchronizing system and a monitor apparatus that can suitably be applied to an intra-car entertainment system.
FIG. 1 of a known intra-car entertainment system 1 adapted to select a set of contents of a program, or an audio and/or video work, coming from a TV broadcasting station or a radio broadcasting station or stored in a DVD (digital versatile disk) or a hard disk 2 by means of a selector 3 according to a controller operation made by a user and transmits the contents data of the selected set of contents to a decoder 4.
The decoder 4 restores the images and the sounds of the contents by decoding the transmitted contents data and causes a display 5 to display the restored images and a speaker 6 to output the restored sounds. Thus, the images and the sounds are output in a synchronized state.
Meanwhile, Japanese Patent Appln. Laid-Open Publication No. 2004-201022 discloses a technique for outputting images and sounds in a synchronized state. More specifically, the sounds of a moving picture are wirelessly transmitted to a remotely located speaker or the like and output in synchronism with the images being displayed on a display apparatus.
On the other hand, so-called mini-vans and other wagon type automobiles that have become popular in recent years are mostly equipped with a wireless rear monitor for providing contents to the passenger(s) sitting on the rear seat for the purpose of entertainment. Then, the images of a set of contents can be wirelessly transmitted from a contents server arranged near the front seats to the wireless rear monitor arranged near the rear seat and displayed there.
Concretely, as shown in FIG. 2, an intra-car entertainment system 10 using a wireless rear monitor includes a wireless rear monitor 12 and is adapted to transmit wirelessly only the video data of a set of contents from a contents server 11 arranged near the front seats to the wireless rear monitor 12 arranged near the rear seat.
Thus, the wireless rear monitor 12 decodes the video data of a set of contents received from the contents server 11 at the side of the front seat and displays the set of contents. The wireless rear monitor 12 can also electrically charge the internal battery it has by means of a charging station 14 that is connected to a power source 13 and also arranged near the rear seat. The wireless rear monitor 12 may be arranged on the charging station 14. It may be needless to say that the wireless rear monitor 12 can display the images of a set of contents while it is arranged on the charging station 14.
FIG. 3 is a schematic block diagram of the intra-car entertainment system 10, in which the components same as those of FIG. 1 are denoted respectively by the same reference symbols. Referring to FIG. 3, contents server 11 of the intra-car entertainment system 10 selects a set of contents from sets of contents including those that are compressed and encoded by an encoder 21 and those that are read out from a hard disk 2 by means of a selector 3 that operates according to the operation of a controller and transmits the selected set of contents to a distributor 22.
The distributor 22 separates the video data and the audio data of the contents data of the set of contents and transmits only the audio data to an audio decoder 23, which decodes the audio data so that the decoded audio data are output from a front speaker 6 as the sounds of the set of contents. On the other hand, the distributor 22 packetizes the video data and wirelessly transmits the packetized video data to a wireless transmitter/receiver 25 of the wireless rear monitor 12 by way of a wireless transmitter/receiver 24.
In the wireless rear monitor 12, the video data received by means of the wireless transmitter/receiver 25 are sent out to a video decoder 26, which video decoder 26 decodes the contents and displays the images of the contents on the display 27.
Referring to FIG. 4, the wireless transmitter/receiver 24 of the contents server 11 is adapted to transmit video data as PS (program stream) packets when it transmits video data to the wireless rear monitor 12. A PS packet is composed of a PS header and a PES (packetized elementary stream) packet.
A start code, SCR bases and an SCR extension are described in the PS header. The SCR bases represent the upper order digit values of an SCR (system clock reference) expressed by upper order 33 bits with a degree of precision of 90 [khz], whereas SCR extension represents the lower order digit values expressed by lower order 9 bits with a degree of precision of 27 [MHz].
Note that an SCR is a time stamp for transmitting clock time information of the transmitting side, or the contents server 11, to the wireless rear monitor 12. The system time clock (to be referred to as STC hereinafter) of the contents server 11 and the STC of the wireless rear monitor 12 are synchronized with each other by means of the SCR.
Referring now to FIG. 5, a PES packet is composed of a PES header and video data of an access unit as defined in the International Organization for Standardization (ISO)/IEC13813-1. A start code, a stream ID, a packet length . . . , a PTS (presentation time stamp) and a DTS (decoding time stamp), . . . are described in the PES header.
A PTS is a clock counter value as observed at a 90 [khz] clock that indicates the output clock time of the access unit of the video data. A DTS is a clock counter value as observed at a similar 90 [khz] clock that indicates the decoding time clock, showing the timing of decoding.
Thus, the video decoder 26 of the wireless rear monitor 12 is adapted to reproduce the STC according to the SCR of the PS packet received from the contents server 11, decode the video data at the time when the clock time of the STC becomes equal to the value of the PTS and that of the DTS value and output the decoded data to the display 27.
In an intra-car entertainment system 10 having the above-described configuration, transmission delays can occur due to various factors such as retransmission control that is required when a transmission error arises because video data that are packetized to PS packets are wirelessly transmitted from the contents server 11 to the wireless rear monitor 12.
More specifically, as shown in FIGS. 6A and 6B, the video decoder 26 of the wireless rear monitor 12 is adapted to decode the video data according to the SCR and the PTS and the DTS of the PES packet contained in the PS packet of a frame (29.97 [Hz]) it receives from the contents server 11 on the basis of a 1 access unit and output the decoded video data to the display 27.
Note that the PTS and the DTS are the values of the readings of clock counters as observed by means of [90 khz] clocks respectively and the SCR is the value of the reading of a clock counter as observed by means of a [27 MHz] clock. The value of the reading of the clock counter of the SCR is expressed by that of a clock counter of frequency division of 1/300. Thus, the video data of a frame is transmitted at every reading of “3003” of the clock counter.
The wireless rear monitor 12 decodes video data and outputs them to the display 27 according to the SCR, the PTS and the DTS it receives from the contents server 11. If, for instance, a transmission delay of 0.5 seconds occurs between the contents server 11 and the wireless rear monitor 12 due to a control operation for retransmitting video data due to a transmission error at the time of the wireless transmission, there arises a time lag of 0.5 seconds in the clock time information of the SCR between the contents server 11 and the wireless rear monitor 12 so that the sounds output from the front speaker 6 by the contents server 11 and the images displayed on the display 27 of the wireless rear monitor 12 would no longer show lip synchronization.
Thus, even when the images that are displayed on the display 27 of the wireless rear monitor 12 show a delay of 0.5 seconds relative to the sounds output from the front speaker 6 by the contents server 11 in an intra-car entertainment system 10, the user watching and listening the set of contents may feel strange and uneasy.
In view of the above identified circumstances, it is therefore an object of the present invention to provide an audio/video synchronizing system and a monitor apparatus that can provide a set of contents where the images and the sounds of the contents show lip synchronization so that the user may not have any strange and uneasy feeling if only the images are transmitted wirelessly.